300
ELEMENTS OF MATERIA MEDICA.
packed in tight barrels, and constitute the pot-ashes of commerce. Itscolour varies somewhat, but it is usually reddish, in consequence of thepresence of sesquioxide of iron.
To make the substance called pearl-ash (potassa impura, Ph . L.)the mass called black salts, instead of being fused, is transferred from thekettles to a large oven-shaped furnace, so constructed that the flame ismade to play over the alkaline mass, which in the meantime is stirred bymeans of an iron rod. The ignition is in this way continued until thecombustible impurities are burnt out, and the mass, from being black,becomes dirty bluish white : this is pearl-ash ( United States Dispensatory).The colouring matter is probably manganesiate of potash.
The following table shows the composition of various kinds of pot-ashand pearl-ash, according to Yauquelin {Ann. de Chirn. xl. 273):—
Kinds of Potash.
CausticHydrateof Potash.
Sulphate
of
Potash.
Chloride
of
Potas-
sium.
Insoluble
residue.
857
154
20
2
119
772
65
5
56
254
American Pearlash .
754
80
4
6
308
Potash of Treves . .
720
165
44
24
199
JJantzic Potash . . .
G03
152
14
79
304
Potash of Vosges . .
444
148
510
34
16
In this table it will be observed, that the American pot-ash containsthe largest quantity of caustic potash: this arises, probably, from theuse of lime in its manufacture. Moreover, pearl-ash contains more car-bonate of potash than pot-ash: this must arise from the absorption ofcarbonic acid during its preparation.— [For the mode of estimating thequantity of alkali present, see Mr. Faraday ’s Chemical Manipulation, art.Alkalimetry.'] The pot-ash and pearl-ash employed in this country areprincipally imported from the British North American colonies, fromRussia, and from the United States of America .
In the Pharmacopoeia carbonate of potash is directed to be prepared asfollows:—Dissolve two pounds of impure carbonate of potash (pearl-ash)in a pint and a half of water, and strain ; then pour it off' into a propervessel, and evaporate the water, that the liquor may thicken ; then stir itconstantly with a spatula until the salt concretes. In this process theearthy impurities insoluble in water are got rid of. The same authorityalso states, that a purer carbonate may be prepared by heating the crys-tals of the bicarbonate to redness.
The high price of pearl-ash has occasionally led to the manufacture ofcarbonate of potash from sal enixum (bisulphate of potash), by heating itin a reverberatory furnace with charcoal. This yields sulphuret of potas-sium, in consequence of the carbon deoxidizing the bisulphate. Byroasting, the sulphuret is decomposed, and converted into the carbonateof potash; the sulphur being dissipated, and the potassium combiningwith oxygen and carbonic acid.
Properties. —Carbonate of potash (the salt of tartar of the shops) isusually kept in a granular condition, on account of the difficulty of crys-tallizing it. It is white, inodorous, and strongly alkaline to the taste. Itreacts powerfully as an alkali on turmeric or infusion of red-cabbage. Itis fusible at a red heat ; has a strong affinity for water, so that by expo-